This invention relates to a sewage treatment system and, more particularly to a wastewater treatment system.
Wastewater treatment systems are well known in the art. Typical wastewater treatment systems generate raw primary sludge and waste activated sludge, which they typically thicken, heat, and digest in anaerobic digesters. Anaerobic digesters are typically operated under mesophilic conditions, from approximately 20° C. to approximately 35° C., and the product from these digesters is typically dewatered to produce Class B sludge. The Class B sludge is typically hauled away to land application or is composted or lagooned to produce a Class A sludge. The National Institute of Occupational Safety and Health (NIOSH) has classified Class B sludge as a biohazard, so the wastewater treatment industry is moving away from producing Class B sludge and toward producing Exceptional Quality Class A sludge (EQ Class A). The present invention combines a number of known elements in new and creative ways with a surprising synergy of mechanical, thermal, and chemical integration to generate EQ Class A sludge at low capital and operating cost.
U.S. Pat. No. 5,221,486, issued in 1993 to Fassbender, U.S. Pat. No. 5,433,868, issued in 1995 to Fassbender, U.S. Pat. No. 5,785,852, issued in 1998 to Rivard et al. in 1998, and U.S. Pat. No. 6,143,176, issued in 2000 to Nagamatsu et al., describe and disclose a number of prior art approaches to wastewater treatment systems. The disclosures of U.S. Pat. Nos. 5,221,486, 5,433,868, 5,785,852, and 6,143,176 are incorporated herein by reference. Waste activated sludge is more difficult to dewater and digest because of its hydrophilic and cellular nature. To address this problem, the '852 patent discloses the use of low temperature heat, in the range from 180° F. to 385° F., and explosive flash and shear forces to disrupt cells so that the soluble material in the cells is released and available for anaerobic digestion. The '868 patent describes a process in which a combined stream of waste activated sludge and primary sludge is treated at high temperature hydrothermal conditions to produce oil, char, and an ammonia containing wastewater stream. The wastewater stream is further processed with another hydrothermal process to convert the ammonia to nitrogen gas. The '176 patent describes the use of a hydrothermal process for heating anaerobically digested sludges to generate a carbon slurry that is dewatered to provide a concentrated carbon slurry of char and oil having a high heating value. The aqueous phase separated from the carbon slurry to form the concentrated carbon slurry is returned for additional anaerobic digestion.
These systems offer a number of advantages in processing wastewater. They generally do a relatively good job of recovering valuable resources from wastes and of reducing the amounts of such wastes that must be sent to landfills. Still, they suffer from a number of disadvantages. For example, because the sludges contain large amounts of water, subjecting both a primary sludge and a waste activated sludge to one or more hydrothermal processes requires a great deal of energy just to heat and cool the water contained therein. Combining the primary sludge and waste activated sludge in an anaerobic digester would result in a large energy demand to heat the anaerobic digester feed, particularly if the anaerobic digester is to be operated under more desirable thermophilic conditions. Combining the primary sludge and waste activated sludge in an anaerobic digester would also tend to force an operator to choose between undesirably increased capital cost or undesirably decreased treatment time. Similarly, combining the primary sludge and waste activated sludge in the anaerobic digester would also force an operator to choose between undesirably increased operating costs for heating or undesirably low operating temperature, perhaps leading to the use of acceptable but less desirable mesophilic conditions rather than thermophilic conditions. Further, the primary sludge typically includes more solids and particulate matter that is hard on equipment operating at high temperature and pressure, such as the conditions typically encountered in hydrothermal processes. Again, heating both the primary sludge and the waste activated sludge to the high temperatures called for in a hydrothermal process requires a great deal of energy. Also, in systems that use aerobic and anoxic zones in a digester to treat the water effluent and generate the waste activated sludge, maintaining optimal conditions for the nitrate reducing and phosphourus accumulating bacteria in the aerobic/anoxic digester typically requires additional raw sewage to be fed into the aerobic/anoxic digester or that a water soluble carbon source such as methanol feed stream be provided. Further still, the sludges often cause clogging or fouling problems, as they are being prepared for and passed to and through hydrothermal processes. This common problem typically leads to the use of scraped surface heat exchangers in an effort to combat or counter such problems. Also, because the primary sludge and waste activated sludge are typically treated together or at similar temperature ranges, there is little or no opportunity for efficient heat transfer between the two to offset operating expenses.